1. Field of the Invention
The present invention relates generally to honeycomb made from aramid fiber paper that is coated with a phenolic resin. More particularly, the present invention is directed to providing alternative aramid fiber-based papers that have reduced aramid fiber content, but which can be coated with phenolic resin to produce an alternative honeycomb material that exhibits structural and thermal properties that are on the same level as conventional honeycomb material made using well known aramid paper.
2. Description of Related Art
A popular type of aircraft structural panel is made up of face sheets (also referred to as “skins”) that are adhesively bonded to opposite sides of a light weight core material These types of sandwich panels are desirable in aircraft because they are light weight and very strong. Honeycomb is widely used as a core material in aircraft structural panels because of its high strength to weight ratio and resistance to fatigue failures.
For example, honeycomb structural panels have been used as floor panels in the fuselages of aircraft. Floor panels are used, especially in large commercial aircraft, to separate the passenger compartment from the various electrical, hydraulic and structural components located in the bottom of the fuselage. The floor panels are generally classified depending on their location and intended use. For example, large commercial aircraft typically include four different types of floor panels: 1) aisle panels; 2) under seat panels; 3) galley panels; and 4) high load panels. The physical characteristics of the panels vary depending on the particular loads experienced during use. For example, the aisle and under seat floor panels tend to be lighter than and not as strong as galley and high load floor panels.
Honeycomb structural panels, as well as other parts and materials, which are used in aircraft interiors, must exhibit certain minimum levels of resistance to heat and fire. The specific thermal requirements for structures, parts and other materials used in aircraft interiors are set forth in 14 C.F.R. Part 25.853.
One requirement under 14 C.F.R. Part 25.853 is that honeycomb structural panels, when oriented in a vertical position, must be “self-extinguishing” after being subjected to a flame for 12 or 60 seconds in accordance with the test procedures set forth in Appendix F of 14 C.F.R. Part 25.853. Another requirement is that the honeycomb structural panels must have certain heat release properties when tested according to standardized Ohio State University (OSU) heat release tests, as set forth in Appendix F of 37 C.F.R. Part 25.853.
NOMEX® aramid paper is a well-established material that has been widely used for many years to make honeycomb for use in aircraft structural panels. NOMEX® aramid paper is produced by E.I. DuPont de Nemours & Company (Wilmington, Del.). Honeycomb cores made from NOMEX® aramid paper that has been coated with a phenolic resin have been, and continue to be, very popular because they provide a desirable combination of lightweight, high strength and excellent thermal properties.
NOMEX® paper, by itself, does not have to meet the requirements of 14 C.F.R. Part 25.853 because the requirements under this regulation are directed to the structural panel, as a whole, as opposed to the individual components that make up the panel. Even so, the individual thermal properties of the face sheets, the honeycomb, and the paper that are used to make the honeycomb structural panels, are still important and must be taken into consideration when making a structural panel for use in the interior of an aircraft. It is desirable that the individual thermal properties of these components be sufficient so that they can be combined to form a structural panel where the panel, as a minimum, will meet the requirements of 14 C.F.R. Part 25.853.
The testing procedures set forth in 14 C.F.R. Part 25.853 may be used to evaluate the thermal properties of the paper and the honeycomb made from the paper in order to provide an indication of the suitability of the paper and honeycomb for use in making a structural panel that meets or exceeds the requirement of 14 C.F.R. Part 25.853. For example, NOMEX® paper, by itself, self-extinguishes in less than 1 second and exhibits a vertical burn length of less than 9 inches (22.9 cm) when subjected to the 12-second vertical burn test set forth in Appendix F. When NOMEX® aramid paper is coated with a phenolic resin, the resulting coated-paper material self-extinguishes in about 2 seconds and exhibits a vertical burn length of about 2 to 3 inches (5 to 7.6 cm).
NOMEX® aramid paper, which has been formed into a honeycomb that is coated with a phenolic resin, exhibits a peak heat release of about 57 kW/m2, a time to peak heat release of about 51 seconds and a 2 minute heat release of about 68 kW-minute/m2 when tested in accordance with Appendix F of 37 C.F.R. Part 25.853.
The above thermal properties make NOMEX® aramid paper particularly well suited for use in aircraft structural panels that meet or exceed the burn requirements set forth in 14 C.F.R. Part 25.853. However, NOMEX® aramid paper, and aramid papers in general that are composed entirely of aramid fibers, tend to be relatively expensive. Accordingly, there is an incentive to develop alternative papers that can be coated with a phenolic resin to provide an alternate resin-coated paper material and formed into honeycomb that is suitable for making structural panels that can be used in aircraft and other demanding applications where excellent thermal properties, such as those exhibited by NOMEX® aramid paper, are required.